The Namaqua Metamorphic Complex: Geochronology and geochemical signatures of the Aus Crystalline Complex (Southern Namibia)

The Namaqua Metamorphic Complex (NMC) forms a basement unit of the Grenvillian-Kibaran collisional orogen in southern Namibia and South Africa. The Aus Crystalline Complex is a terrane within the NMC and consists, predominantly, of gneisses, granites, metavolcanites, and metasediments. The objective of our study is to generate new knowledge about crustal evolution during the Kibaran orogeny using age dating and whole rock geochemistry of various basement units of the crystalline complex around Aus.

Outcropping basement rocks were Aampled in the Namib Desert, southwestern Namibia, east of the coastal town of Lüderitz, within a 20 km radius around Aus. Rock types encountered included granulites, gneisses, charnockites, granites, and pegmatites. Geochemical and geochronological analyses were performed to gain information on the timing of crystallisation and crust production processes, using U-Pb dating on magmatic zircons with LA-ICP-MS. Analysed zircon grains are often complex, U-Pb ages range from 1700 Ma to c. 700 Ma. The oldest rock is a charnockite showing an age of intrusion of c. 1700 Ma, which represents a Palaeoproterozoic part of the cratonic basement. U-Pb zircon ages of the gneisses point to protolith ages of c. 1155 Ma – 1145 Ma. This coincides with a major tectono-magmatic event in the NMC, which occurred in a time span of c. 1190 - 980 Ma. Such processes led to the amalgamation of the supercontinent Rodinia in late Mesoproterozoic time. A younger suite of granitoids (c. 700 Ma) is most probably related to Rodinia dispersal.